Machines of this kind have become known in a variety of constructions. An external after-pressure piston is particularly used, if the after-pressure (the pressure after shooting or injecting the material to be shaped into the cavity) of the ram is no longer sufficient to compensate for all shrinkage occurring in the cavity. This will generally be the case with all complicated dies or molds and with complicated shaped parts.
Just with such complicated dies or molds where some sections present a particularly high flow resistance, while other sections do not, there will be still another problem: Such cavities are difficult to evacuate efficiently. Certainly it has been suggested to provide one vacuum valve at the entrance of the cavity and another one at the end of the cavity, the valves serving to control sucking off the air contained in the cavity prior to inserting (“shooting” or injecting) the material to be shaped. This method, however, is not efficient with all types of cavities.